A conventional image forming apparatus of the electrophotographic type has been widely used as a copying machine, a printer, a plotter, a facsimile machine, a multi-function machine having a plurality of functions of these machines, and the like. In the image forming apparatus of this type, charged toner is brought near to a photosensitive drum and is electrostatically deposited on an electrostatic latent image on the photosensitive drum, and thus development is carried out, so that an image is formed. As a developing type, in addition to a developing type using a one-component developer, as a developer, consisting of magnetic toner, a developing type using the two-component developer in which the non-magnetic toner and the magnetic carrier are mixed with each other has also become widespread. According to the developing type using the two-component developer, this developing type is excellent in stability of a toner charge amount and therefore it is possible to form a color image excellent in color tint, and particularly, the developing type is suitably applied to an image forming apparatus of a color image.
In the developing type using the two-component developer, the developer is carried on a developing sleeve by a magnet (magnetic field generating means) fixedly provided in the developing sleeve and the magnetic carrier forms magnetic chains along magnetic lines of force of the magnetic field generating means. When the developer is fed to a region in which the developing sleeve is close to the photosensitive drum, the magnetic chains contact the photosensitive drum. Thereafter, the magnetic chains are separated from through a region in which the developing sleeve is closest to the photosensitive drum. This region from contact of the magnetic chains with the photosensitive drum to separation of the magnetic chains from the photosensitive drum is a contact nip, and herein, this contact nip is referred to as a developing region. Further, principally in this region, the toner is deposited on the electrostatic latent image by a force of an electric field generated by a potential difference between the developing sleeve and the electrostatic latent image on the photosensitive drum, so that a toner image is formed.
In the developing type using the two-component developer, it is important that a toner development amount per potential difference between an exposure potential on the photosensitive drum and the developing sleeve, i.e., a so-called developing efficiency is increased. When the developing efficiency is low, there is a need that in order to provide a sufficient image density, the potential difference between the exposure potential and the developing sleeve is made larger and thus the electric field strength is enhanced and thereby to increase the toner development amount. However, when the electric field strength is excessively enhanced, there is a liability that the carrier of the two-component developer is deposited together with the toner on the photosensitive drum. The carrier deposited on the photosensitive drum impairs transfer of the toner and causes an occurrence of white dropout (white void) on an image. For that reason, there is a need to increase the toner development amount without enhancing the electric field strength.
As a method of enhancing the developing, there is a method of extending a developing region. In order to extend the developing region, in the region in which the developing sleeve is close to the photosensitive drum, an erected chain region of the developer may only be required to be increased. In order to increase the erected chain region of the developer, a developing device in which a half peak width of a developing magnetic pole, opposing the photosensitive drum, of a plurality of magnetic poles constituting the magnetic field generating means fixedly disposed in the developing sleeve is made large has been known (Japanese Laid-Open Patent Application (JP-A) 2001-34067).